Hearing loss with a mitochondrial gene mutation is highly prevalent in Japan

Postlingual hearing loss as a mitochondrial 3243A>G mutation phenotype

Background

The prevalence of isolated hearing loss (HL) associated with the m.3243A>G mutation is unknown. The aim of this study was to assess the frequency and heteroplasmy level of the m.3243A>G mutation in a large group of Polish patients with postlingual bilateral sensorineural HL of unidentified cause.

Methodology/Principal Findings

A molecular search was undertaken in the archival blood DNA of 1482 unrelated patients with isolated HL that had begun at ages between 5 and 40 years. Maternal relatives of the probands were subsequently investigated and all carriers underwent audiological tests. The m.3243A>G mutation was found in 16 of 1482 probands (an incidence of 1.08%) and 18 family members. Of these 34 individuals, hearing impairment was detected in 29 patients and the mean onset of HL was at 26 years. Some 42% of the identified m.3243A>G carriers did not develop multisystem symptomatology over the following 10 years. Mean heteroplasmy level of m.3243A>G was lowest in blood at a level of 14% and highest in urine at 58%. These values were independent of the manifested clinical severity of the disease.

Conclusions

A single m.3243A>G carrier can usually be found among each 100 individuals who have postlingual hearing loss of unknown cause. Urine samples are best for detecting the m.3243A>G mutation and diagnosing mitochondrially inherited hearing loss.

The responsible genes in Japanese deafness patients and clinical application using Invader assay

Discovery of deafness genes has progressed but clinical application lags because of the genetic heterogeneity. To establish clinical application strategy, we reviewed the frequency and spectrum of mutations found in Japanese hearing loss patients and compared them to those in populations of European ancestry. Screening revealed that in Japanese, mutations in GJB2, SLC26A4, and CDH23, and the mitochondrial 12S rRNA are the major causes of hearing loss. Also, mutations in KCNQ4, TECTA, COCH, WFS1, CRYM, COL9A3, and KIAA1199 were found in independent autosomal dominant families. Interestingly, spectrums of GJB2, SLC26A4, and CDH23 mutations in Japanese were quite different from those in Europeans. Simultaneous screening of multiple deafness mutations based on the mutation spectrum of a corresponding population using an Invader panel revealed that approximately 30% of subjects could be diagnosed. This assay will enable us to detect deafness mutations in an efficient and practical manner in the clinical platform. We conclude that specific racial populations may have unique deafness gene epidemiologies; therefore, ethnic background should be considered when genetic testing is performed. Simultaneous examination of multiple mutations based on a population's spectrum may be appropriate and effective for detecting deafness genes, facilitating precise clinical diagnosis, appropriate counseling, and proper management.

Mitochondrial non-syndromic sensorineural hearing loss: a clinical, audiological and pathological study from Italy, and revision of the literature

Over the last decade, a number of distinct mutations in the mtDNA (mitochondrial DNA) have been found to be associated with both syndromic and non-syndromic forms of hearing impairment. Their real incidence as a cause of deafness is poorly understood and generally underestimated. Among the known mtDNA mutations, the A1555G mutation in the 12S gene has been identified to be one of the most common genetic cause of deafness, and it has been described to be both associated to non-syndromic progressive SNHL (sensorineural hearing loss) and to aminoglycoside-induced SNHL. In the present study, we have investigated the presence of mtDNA alterations in patients affected by idiopathic non-syndromic SNHL, both familiar and sporadic, in order to evaluate the frequency of mtDNA alterations as a cause of deafness and to describe the audiological manifestations of mitochondrial non-syndromic SNHL. In agreement with previous studies, we found the A1555G mutation to be responsible for a relevant percentage (5.4%) of cases affected with isolated idiopathic sensorineural hearing impairment.

Mitochondrial syndromic sensorineural hearing loss

Mitochondrial diseases (MD) are a clinically heterogeneous group of disorders that arise as a result of dysfunction of the mitochondrial respiratory chain. Sensorineural hearing loss (SNHL) is often associated to mitochondrial dysfunctions both in syndromic, nonsyndromic forms. SNHL has been described in association to different mitochondrial multisystemic syndromes, often characterized by an important neuromuscular involvement. Because of the clinical relevance of the associated neurological symptoms, the occurrence of SNHL is often underestimated and undiagnosed. In this study we evaluated the incidence of SNHL in a group of 17 patients with MD. We detected some degree of hearing impairment in 8/17 patients (47%), thus confirming the frequency of hearing impairment in MD. Furthermore, we want to highlight the role of the audiologist and otolaryngologist in the diagnosis and characterization of a MD, which should be suspected in all the cases in which the hearing loss is associated to signs and symptoms characteristic of mitochondrial dysfunction, especially if the family history is positive for hearing loss or MD in the maternal line.

Mitochondrial encephalopathy with lactic acidosis and stroke-like episodes syndrome with hypothyroidism and focal segmental glomerulosclerosis in a paediatric patient

Herein, we report on a paediatric patient with mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) who was hospitalized for acute on chronic renal insufficiency, seizures and deterioration of the level of consciousness. She also had hypertension, hypothyroidism and nephrotic range proteinuria. Kidney biopsy revealed many sclerotic glomeruli and focal segmental glomerulosclerosis (FSGS). Glomerulopathy is rare in patients with MELAS, and FSGS has been reported only in a few patients. The histopathological features of the renal biopsy suggested that the aetiology of the FSGS may have been secondary to chronic renal injury rather than from a primary immunologic cause. Moreover, our case is unique in that, the coexistence of MELAS, hypothalamic hypothyroidism and FSGS has not been reported in the past. The purpose of this report is to increase the awareness of health-care professionals, especially in the fields of paediatrics, neurology, endocrinology and nephrology, regarding the manifestations and complications of MELAS.

Gene-Based Deafness Research: Ion Transport and Hearing

The cochlea is a sensory organ that converts physical (sound) stimulation into electrical signals. This process is fundamentally and substantially based upon the ion transport system. Here, I summarize the physiological and molecular biological aspects of transporters, channels and receptors expressed in the cochlea. With reference to these findings, recent advances in genetic research on hereditary deafness are discussed.

Characterization of novel and identified genes in guinea pig organ of corti

A number of proteins are expressed in the organ of Corti and are considered to be responsible for hearing. However, most of them have not been identified. Therefore, to achieve a better understanding of the genetic factors influencing these traits, the first step is to characterize the genes expressed in the organ of Corti. In the present study, a cDNA library was constructed from the guinea pig organ of Corti. After sequencing isolated clones, 196 expressed sequence tags (ESTs) were identified with FASTA analysis: 65 ESTs showed significant sequence homology to previously identified genes in guinea pig, human or other species, and 131 ESTs showed no significant matches to sequences already present in the DNA database DDBJ/GenBank/EMBL. A variety of matching sequences, some of which were known to be cochlea-specific, were found through FASTA analysis of the 65 clones. RT-PCR with a panel of 10 different tissue mRNA revealed the restricted expression of 13 unknown clones. The results of our analysis allowed the establishment of a list of genes expressed in the guinea pig organ of Corti.

The spectrum of hearing loss due to mitochondrial DNA defects

Heteroplasmic mitochondrial DNA (mtDNA) defects are an important cause of neurological disease. Although hearing impairment is common in patients with mtDNA defects, the spectrum and pathophysiology of the hearing loss is not well characterized. We therefore studied the relationship between cochlear and brainstem auditory function in 23 patients harbouring a range of different mtDNA mutations. Based upon the pure tone audiogram, patients fell into three distinct groups: (i) normal hearing, (ii) mild to moderate predominantly high frequency hearing loss, and (iii) severe or profound hearing loss at all frequencies. Within this study group only certain genetic defects were associated with hearing loss, and for individuals harbouring the A3243G point mutation, the severity of the hearing loss correlated with the percentage level of mutated mtDNA (mutation load) in skeletal muscle. The 10 patients who had a moderate hearing loss or less had normal brainstem auditory evoked responses and MRI, but it was not possible to interpret the brainstem auditory evoked responses in 13 patients with severe hearing loss. Otoacoustic emissions were absent in patients with a moderate or more severe hearing loss. These findings are consistent with a predominantly cochlear origin for the hearing deficit, which is determined by the precise genetic defect and the percentage mutation load.